5 α-Amino Acids In Water: A Review of VCD and ROA Spectra
103
charge interaction with the medium is calculated. the cavity is differently defined
in the various versions of models (a sphere, an ellipsoid, or a more complicated
shape following the surface of a molecule). the cavity should not contain solvent
molecules but rather contain, within its boundaries, the solute charge distribution. It
should be noted that a drawback of the model is that the results are quite sensitive
to the cavity radius. the solvent reaction potential can be partitioned into several
contributions of different physical origin, related to electrostatic, repulsive, induction, and dispersion interactions between the solute and solvent.
the most popular of these choices are:
1. PCm methods developed primarily by the Pisa group of tomasi and co-workers
[115, 117, 124], in which only the electrostatic and induction terms are explicitly
considered as the interaction potential v R . the cavity is formed from spheres
centred on nuclei in the solute molecule. the electrostatic part of the solventsolute interaction is represented by the charge density spread on the cavity surface (Apparent Surface Charges, ASC). In the original method, called dielectric
d-PCm, the magnitude of the point charges was determined on the basis of the
dielectric constant of the solvent.
Now, the Integral Equation Formalism (the IEF-PCm method), by Cancès, mennucci and tomasi is the most popular PCm version [125]. It employs a moleculeshaped cavity composed of spheres centred on the nuclei, while the reaction-field
is modelled by placing charges on the cavity surface. to characterise the intermolecular interactions, it is necessary to take into account the non-electrostatic
terms. Amovilli and mennucci have described an approach where the repulsion
and dispersion terms are computed self-consistently as part of the reaction field
operator [126]. the IEF-PCm method performs better than the reaction-field
one, because a more realistic cavity shape is taken into account.
2. the second approach is conductor C-PCm of Barone and Cossi [127], in which the
surrounding medium is modelled as a conductor instead of a dielectric medium.
3. the Conductorlike Screening model (CoSmo), a modification of the C-PCm
method proposed by Klamt and coworkers [128–130], is also extensively used.
As suggested by the name, in the CoSmo model the solvent is modelled as a
conductor while the cavity shape is the same as in the IEF-PCm method.
4. Several other PCms, i.e., the multipole Expansion method (mPE) by the Nancy
group of Rivail and Rinaldi [118, 131] and by mikkelsen and co-workers [132,
133] and the generalised Born Approximation (gBA) by the minneapolis group
of Cramer and truhlar [134–136], have also been introduced.
over the years, a number of PCm models have been used for the calculations and
interpretations of the vCd and RoA spectra, differing in the shape of the cavity
and in the treatment of the solute-solvent interaction. the majority of PCm calculations of solvent effects on vCd and RoA spectra employ either the IEF-PCm or
CoSmo approaches. Nevertheless, when the specific solvent-solute interactions
become important, the continuum methods are no longer appropriate. In such cases,
the explicit supermolecular method in conjunction with the implicit PCm approach
should be used.
103
charge interaction with the medium is calculated. the cavity is differently defined
in the various versions of models (a sphere, an ellipsoid, or a more complicated
shape following the surface of a molecule). the cavity should not contain solvent
molecules but rather contain, within its boundaries, the solute charge distribution. It
should be noted that a drawback of the model is that the results are quite sensitive
to the cavity radius. the solvent reaction potential can be partitioned into several
contributions of different physical origin, related to electrostatic, repulsive, induction, and dispersion interactions between the solute and solvent.
the most popular of these choices are:
1. PCm methods developed primarily by the Pisa group of tomasi and co-workers
[115, 117, 124], in which only the electrostatic and induction terms are explicitly
considered as the interaction potential v R . the cavity is formed from spheres
centred on nuclei in the solute molecule. the electrostatic part of the solventsolute interaction is represented by the charge density spread on the cavity surface (Apparent Surface Charges, ASC). In the original method, called dielectric
d-PCm, the magnitude of the point charges was determined on the basis of the
dielectric constant of the solvent.
Now, the Integral Equation Formalism (the IEF-PCm method), by Cancès, mennucci and tomasi is the most popular PCm version [125]. It employs a moleculeshaped cavity composed of spheres centred on the nuclei, while the reaction-field
is modelled by placing charges on the cavity surface. to characterise the intermolecular interactions, it is necessary to take into account the non-electrostatic
terms. Amovilli and mennucci have described an approach where the repulsion
and dispersion terms are computed self-consistently as part of the reaction field
operator [126]. the IEF-PCm method performs better than the reaction-field
one, because a more realistic cavity shape is taken into account.
2. the second approach is conductor C-PCm of Barone and Cossi [127], in which the
surrounding medium is modelled as a conductor instead of a dielectric medium.
3. the Conductorlike Screening model (CoSmo), a modification of the C-PCm
method proposed by Klamt and coworkers [128–130], is also extensively used.
As suggested by the name, in the CoSmo model the solvent is modelled as a
conductor while the cavity shape is the same as in the IEF-PCm method.
4. Several other PCms, i.e., the multipole Expansion method (mPE) by the Nancy
group of Rivail and Rinaldi [118, 131] and by mikkelsen and co-workers [132,
133] and the generalised Born Approximation (gBA) by the minneapolis group
of Cramer and truhlar [134–136], have also been introduced.
over the years, a number of PCm models have been used for the calculations and
interpretations of the vCd and RoA spectra, differing in the shape of the cavity
and in the treatment of the solute-solvent interaction. the majority of PCm calculations of solvent effects on vCd and RoA spectra employ either the IEF-PCm or
CoSmo approaches. Nevertheless, when the specific solvent-solute interactions
become important, the continuum methods are no longer appropriate. In such cases,
the explicit supermolecular method in conjunction with the implicit PCm approach
should be used.